This invention relates generally to photonic waveguides, and more particularly relates to techniques for compensation of both microfabrication variation and operational thermal fluctuations in photonic waveguides.
The increasing demand for reduction in electronics power consumption, together with the increasing demand for communication bandwidth, has led to a photonic system paradigm based on the dense monolithic integration of electronic and photonic devices. In this paradigm, the integrated electronics provide computational functionality local to the integrated photonics for communication. Ultra large scale integrated (ULSI) platforms in which photonic devices are microfabricated in conjunction with monolithic electronic devices therefore bridge the gap between conventional electronics, such as CMOS electronics, and fiber optic technology, and can offer solutions to the challenges of both energy consumption and communication bandwidth associated with conventional photonic and computational systems.
Silicon-based microphotonic materials are a compelling choice for electronic-photonic integration platforms, enabling the use of established CMOS fabrication technology as well as seamless electronic-photonic device integration through monolithic integration. In addition, communication bandwidth density advantages can be achieved through the use of wavelength division multiplexing (WDM) silicon-based photonic device architecture. In addition, silicon-based photonics are characterized by strong light confinement, due to high refractive index contrast, that enables photonic integrated circuits (PICs) with large bandwidth, high selectivity, and an ultra-small footprint. The resulting performance benefits of improved power efficiency, increased functionality, enhanced reliability, and reduced cost provide strong incentives for a silicon-based, monolithic electronic-photonic device integration platform.
In the development of densely integrated silicon-based electronic-photonic device platforms, there have been discovered two significant challenges to the successful production and operation of such platforms. First, it has been found that the high refractive index contrast of silicon-based photonic materials, in combination with the sub-micrometer waveguide dimensions employed for integrated silicon-based photonics, result in a strong sensitivity to microfabrication tolerances. For example, for a single-mode silicon-based photonic waveguide, a deviation of only one nanometer from the waveguide design width to the microfabricated waveguide width can cause an effective refractive index variation of as much as 2×10−3, producing a frequency shift of about 100 GHz in the spectral response of an interferometric device. Thus, even for the most advanced and highly accurate microfabrication processes, integrated photonic device requirements impose a severe process constraint, especially for resonant devices, as well as multi-stages and high-quality-factor devices.
Secondly, it is found that during the operation of densely-integrated electronic-photonic platforms, there can be produced significant local as well as global temperature excursions across the platform, e.g., a CMOS chip, and that such temperature excursions can prove problematic for silicon-based photonics. This sensitivity to thermal change is a result of the high thermo-optic coefficient of such materials. In particular, variations in the thermo-optic (TO) index of photonic materials can induce temperature-dependent wavelength shifts (TDWS) of, e.g., resonant photonic devices, that can limit wavelength resolution in important photonic applications such as WDM, high-resolution spectroscopy, and other applications.
With these limitations, there remain significant challenges to achieving the functionality and operational performance required of monolithically-integrated electronic-photonic device platforms. Unavoidable monolithic microfabrication inaccuracies, coupled with demanding operational conditions, require consideration of both microfabrication processes as well as device design. Successful electronic-photonic integration on a CMOS platform cannot be fully realized without the achievement of such.